Varying the ratio of two Slater integrals ($F^4/F^2$)

The general DFT+$U$ scheme by setting scf.DFTU.Type=2 requires the generation of Coulomb interaction tensor using two input values, $U$ and $J$. This can be done by the combinations of Slater integrals ( $F^0, F^2, F^4,...$) and Racah-Wigner coefficients assuming atomic sphericity. For $d$-orbitals in the standard DFT+$U$ scheme, $F^4/F^2$ ratio should be specified, and $F^4/F^2=0.625$ is the usual choice. This ratio is, however, valid only in the atomic environment and deviates from it in the solid. Users can manually choose the ratio for their own purposes by the following keyword:
  scf.Slater.Ratio                 0.75	   # default=0.625